Space systems for anticipated missions will have very high power requirements due to their complexity and size. For instance, one reactor power system that is being considered would employ thermoelectric converters producing a total of 300 electrical kilowatts. This is three orders of magnitude above the power level of thermoelectric generators used in space thus far.
To help reduce the size, weight and cost of such large thermoelectric power systems, it is advantageous to concentrate the many thermoelectric elements in a relatively small number of rather large thermoelectric converter assemblies, if this can be done without sacrificing reliability. However, in previously proposed thermoelectric converter assembly designs, the end faces of the thermoelectric elements are bonded to rigid hot and cold plates. The principal problem with such design is the large differential thermal expansion of the hot and cold plates due to their large temperature difference, which typically is on the order of 500.degree. K.. If we assume that the plates are made of niobium, this temperature difference produces an expansion mismatch of some of 0.5%. This mismatch, extending over the entire assembly length and width, would produce destructive shear stresses and tensile stresses in the relatively fragile thermoelectric legs and the many interface bond within each cell. To avoid such destructive stresses, compliant elements to accommodate the expansion mismatch must be provided or some other way found to accommodate the shear and tensile stresses resulting from expansion and contraction due to temperature differences.
A thermoelectric converter is usually made up of large number of small, electrically interconnected thermoelectric cells. In the past it has been proposed that the problem of accommodating the large shear and tensile stresses resulting from expansion and contraction due to temperature difference between the hot and cold panels be solved by inserting a compliant pad between each thermoelectric cell and its adjoining hot and/or cold plate. It was proposed that such a compliant pad be made from an array of flexible graphite or metal fibers bonded to two face plates. Such compliant pads could accommodate the thermal expansion differences within each cell, but it is very doubtful that such pads could accommodate the much larger cumulative mismatch between large hotplates and coldplates extending over the entire converter assemblies. Moreover, such fiber pads would also produce substantial temperature drops, which would result in a significant reduction in thermoelectric power and conversion efficiency.
The invention overcomes the thermal stress problems associated with large-scale thermoelectric converters and does not require large non-productive temperature drops or reduced thermoelectric power and conversion efficiency. In addition the invention does not require modification of the individual unit thermoelectric cell such as by the addition of a compliant pad.